The principle of scanning objects with high energy radiation such as x-rays or gamma-rays is widely employed in circumstances where it might be desirable to gain information about the internal contents and/or composition of an object, including for example scanning for identification purposes, for stock control purposes, to monitor changes and especially degradation over time, in a security or like situation for the detection of dangerous or prohibited materials, for example to screen persons or baggage for entry into or exit from a restricted area, for quality control purposes or the purposes of determining the integrity of the structure, or the like.
Most objects that are x-rayed consist of a variety of materials, compounds tissues etc. They are also usually three dimensional in nature but despite this ordinary radiographs are simply flat shadowgraph images which contain only monochrome intensity information. X-ray sources are almost always broad spectrum emitters of radiation. However most detector types are unable to resolve this energy information. Radiographs produced in this way have therefore all the spatial information and all the energy information collapsed into a single 2-D plane. This means that if one is interested in only seeing information inside an object at a particular depth and consisting of a certain type of material, this will always be set against a backdrop of clutter in the image caused by objects in the same x,y region and consisting of other types of materials.
Recent development of detectors that can resolve spectroscopic information about the transmitted x-rays more effectively has led to the development of apparatus that discriminate across a plurality of energy bands to generate a plurality of energy-differentiated images. For example U.S. Pat. No. 5,943,388 describes a system that makes use of cadmium telluride detectors to image across at least three energy bands and generate at least three images. Radiographs produced in this way have some differentiation energy information but all the spatial information remains collapsed into a single 2-D plane.
A method and apparatus have been proposed in application No PCT/GB2008/001103 for the production of monocular movement parallax x-ray images. The architecture of the system must be such that the series of images so produced contain only disparity information in the horizontal direction (i.e. the direction of the separation of the human eyes). Presenting this sequence of images to a human observer will give the same effect as rotating the object. This series of perspective images have contained within them three dimensional coordinate data. In much the same way that photogrammetry can be applied to stereoscopic pairs of images by measuring disparity between perspectives views, so too can it be applied to the series of monocular movement parallax images. A big advantage of doing this is that the more images there are and the greater the disparities involved, then the more accurately the depth or z coordinate can be calculated. A limitation of using stereo pairs is that there is a very tightly controlled limit on the disparity that can be used which effectively limits the depth resolution that can be obtained.
Nevertheless, it remains desirable to develop a method and apparatus for the collection and interpretation of radiographic data that makes effective use of both the spatial information and the energy information collected radiographically by selective energy detectors in a resolved co-ordinated manner.